1. Field of the Invention
The present invention generally relates to implantable medical devices, such as cardiac pacemakers and implantable cardioverter/defibrillators, and in particular to an improved method and medical device for automatically classifying hemodynamic sensor signals.
2. Description of the Prior Art
Today, in the modern society, heart diseases and/or conditions leading to an impaired heart function are a major problem entailing constantly increasing costs for medical services. For example, heart failure is a condition which affects thousands of people throughout the world. Congestive heart failure (CHF) is an ability of the heart to pump blood at an adequate rate in response to the filling pressure. Patients suffering from CHF are often afflicted by cardiogenic pulmonary edema, which is caused by the accumulation of fluid in the lung interstitium and alveoli due to the fact the left ventricular venous return exceeds left ventricular cardiac output. That is, more fluids are transported to the lung region than from the lung region causing the accumulation of fluids in the lung region. CHF may even, in its more severe stages, result in death.
Accordingly, reliable and accurate information, e.g. obtained by means of hemodynamic sensors implanted in the body of a patient, regarding the cardiac function of a patient is of a high value, for example, in order to detect CHF on an early stage or to trend a progression of CHF of a patient. These sensors may include sensors for sensing a blood pressure of the patient or sensing the electrical bio-impedance of the patient. The sensor signals reflects the contraction of the heart, which is highly dependant on whether the patient is paced or not and also the sequence and timings with which the patient is being paced.
One obvious way of increasing the accuracy of the sensor signals is to collect the signals for a number of heartbeats and make a template or reference of the sensor signal representing an average of the heart cycles. Each template hence represents a number of cardiac waveform morphologies. Thereby, influence from noise can be reduced and effects of respiration can be smoothened out. However, when creating such a template from a number of sensor signals for a number of heart cycles it is a great importance that heart cycles that are included into the creation of the template have the same sequence pattern, i.e. the same paced or sensed sequence of heart events. It is also of importance that the starting points of recording the heart beats are the same event in the heart cycle, e.g. a P-wave, R-wave in the right or left ventricle or a pacing pulse in one of the heart chambers.
United States Patent Application Publication No. 2004/0243014 discloses a method and system for creating and maintaining such cardiac waveform templates. A cardiac waveform is formed by identifying one or more cardiac waveform features representative of a particular cardiac beat morphology including morphological features such as curvature, inflection points, rise or fall times, slopes, or the like. Targets regions associated with the identified cardiac waveform features are defined and used to establish a template representing a particular waveform morphology, such as a normally conducted cardiac rhythm. This method thus requires extensive signal processing in order to identify the morphological features such as curvature, inflection points, rise or fall times, slopes, or the like.
United States Patent Application Publication No. 2003/0181818 discloses a method and system for generating a snapshot representative of one beat of a patient's supraventricular rhythm. A number of templates are provided and selectively updated with qualified beats and are used to characterize the patient's supraventricular rhythm.
Accordingly, there is a need of an improved method and medical device for automatically classifying or qualifying hemodynamic sensor signals and for creating hemodynamic sensor signal templates.